130 | Mr. W. K. Brooks on the 
two tubes soon lose their communication with the exterior, 
and the median cloacal aperture is an independent opening 
which is formed later. A-fter the pharynx is formed each 
perithoracic tube unites with it to form a gill-slit. Finally, 
after the perithoracic system is completely outlined its follicu- 
lar cells degenerate and are gradually replaced by blastomeres. 
Our knowledge of the perithoracic system of Salpa in both 
the solitary and the aggregated form is in great confusion. 
Salensky has described the origin of the “ gill” and of the 
median atrium or cloaca of the embryo in a number of species ; 
but the reader of his papers will search in vain for any basis 
of comparison with other Tunicata, or even for any funda- 
mental unity in his account of the various species of Salpa, 
and his papers contain internal evidence that he has misin- 
terpreted his observations. 
Uljanin holds that the perithoracic structures of Doliolum 
are not homologous with those of the ascidian, and Salensky 
holds the same view regarding Salpa. He says that the 
“ oill”’ is part of the body-cavity which is shut in by folds in 
the walls of the pharynx, and that the cloaca is not an inde- 
pendent chamber, but a part of the pharynx which is shut off 
by these folds. A careful study of his description, especially 
pages 119, 200, 224, 225, and 229 of his first paper, and 
pages 114, 139, 160, 163, 838, 839, and 354 of his second 
paper, will show that his views not only involve this conclu- 
sion, but that they would also force us to believe that the 
‘ oill”’ and cloaca of one species of Salpa are not homologous 
with the same structures in another species; for his aecount 
of their origin in Salpa democratica and Salpa pinnata has 
almost nothing in common with his account of them in Salpa 
africana, Salpa pectinata, and Salpa fusiformis. 
In his first paper on Salpa democratiea he says that, like 
Leuckart, he regards the gill as part of the inner mantle or 
branchial sac, that in origin it is nothing more than a strongly 
developed ridge or thickening on the middle line of the dorsal 
surface of the pharynx, and that on each side of it the cavity 
of the pharynx is pushed upwards to form a pair of pouches, 
which soon meet and unite above the cloaca. In this way 
the gill-ridge is transformed into a rod, and the rod, which is 
at first solid, becomes tubular by the conversion of its axial 
cells into blood-corpuscles. 
In this account of the origin of the perithoracic structures of 
Salpa democratica the only point of agreement with my own 
observations on Salpa pianata is his statement that the gill 
is at first solid, and that its central cells are set free as deve- 
lopment progresses. In his second paper he retracts this 
